The invention relates to microspheroidal inorganic particles having increased attrition resistance, and particularly relates to increased attrition resistance in microspheroidal inorganic support particles used for fluidized bed catalysts.
Inorganic microspheroidal particles, especially silica-based microspheroidal particles, are useful for a variety of purposes, including catalyst supports such as in fluidized bed catalysts, as well as for other uses. In many applications, increased attrition resistance of these inorganic microspheroidal particles is beneficial. For example, increased attrition resistance is important in the use of inorganic microspheroidal particles as catalyst supports in a fluidized bed system in which a typical fluidized bed reactor process imparts high stress upon catalyst particles. Typically, during fluidized bed operation, catalyst particles are suspended and circulate in a gaseous fluid and impact upon each other and the sides of the reactor, causing attrition. Attrition results in the formation of smaller particles which typically are removed by filtration as waste in the gas recycle system. This waste of catalytic material may be reduced through using improved attrition resistant inorganic microspheroidal particles. Reduction in the formation of these small particles is desirable especially if the catalyst incorporates costly components such as precious metals. Additionally, these small particles are difficult to remove and may cause plugging of reactor gas recycle lines.
Recently, a fluidized bed catalyst system has been described that is especially useful for fluidized bed vinyl acetate production. U.S. Pat. No. 5,591,688, incorporated by reference herein, describes a microspheroidal fluidized bed catalyst support system formed by spray drying a slurry mixture of an inorganic oxide sol with inorganic oxide particles. The preferable inorganic oxide used in these catalyst supports is silica. In the typical production of these supports, small particles (fines) of the support material are formed and are removed from the support production process as waste. A use for these waste fines would be beneficial to reduce or eliminate a process waste stream. A use of these fine particles that would improve the attrition resistance of the finished catalyst support particle would be especially beneficial.
Catalyst materials using microspheroidal particles as catalyst supports conveniently may be used in a variety of chemical reactions to manufacture various useful products. Being particulate, such catalyst material is especially suitable for use in a fluidized bed reactor. In the operation of a fluidized bed reactor, catalyst material is brought to a fluidized state by suspending and circulating the catalyst material throughout the reactor with a fluidizing medium, typically a gas. While in this fluidized state, the catalyst material is contacted with reactants, and upon contact, the reactants form a desired product. Fluidized bed reactors are often preferable over other types of reactors, such as fixed bed reactors, because fluidized bed reactors are typically better at facilitating a more constant reaction temperature, especially for exothermic reactions. Additionally, fluidized bed reactors typically enable a more thorough and more uniform contact of reactants with the catalyst material.
Unfortunately, one disadvantage of a fluidized bed reactor is the stress placed upon the catalyst material during reactor operation. While in a fluidized state, the catalyst material is impacted upon itself and the reactor walls, causing it to fracture and break, in a process known as attrition. During operation of a fluidized bed reactor, attrition of catalyst material causes the production of attritted particles, which, because of their smaller size, are susceptible to being removed from the reactor along with a desired product. Removal of these attritted particles represents a loss of catalyst material from the reactor, thereby requiring its replenishment. Such loss is particularly undesirable when the catalyst materials contain catalytically active components which are very costly, e.g. gold, palladium, or other precious metals. Furthermore, because attritted particles are removed from the reactor along with the product, a filter is typically required to separate these particles from the product. In some cases, the filter may become clogged with attritted, particles thereby necessitating shutdown of the reactor so that the filter may be cleaned or replaced.
It would therefore be desirable to reduce the problems associated with the attrition of catalyst material, especially during its use in a fluidized bed reactor. In particular, it would be desirable to discover a way to impart durability to a catalyst material so that attrition is reduced. This desired quality of durability is herein termed “attrition resistance”. Catalyst material having attrition resistance is desirable because of its increased useful lifetime and the reduction of catalyst loss.
The invention provides for microspheroidal particles having improved attrition resistance. The microspheroidal particles of this invention are referred to as “attrition resistant microspheroidal particles”. The invention also provides for catalyst material having improved attrition resistance. The catalyst material of this invention is referred to as “attrition resistant catalyst material” and has attrition resistance imparted from incorporation of attrition resistant microspheroidal particles as a catalyst support. The present invention also provides a method of operating a reactor wherein attrition of catalyst material is reduced by use of attrition resistant catalyst material. A reactor operated in accordance with this invention generates a lower amount of attrited particles. Attrition resistant catalyst material is particularly useful in a fluidized bed reactor where formation of attrited particles is an undesirable source of catalyst loss. In addition to catalyst conservation, use of attrition resistant catalyst material reduces clogging of filters designed to separate attrited particles from the product of a fluidized bed reactor.
Attrition resistant microspheroidal particles may be obtained by spray drying a slurry comprising an inorganic sol, an inorganic non-sol, and an attrition modifier. The term “inorganic sol” refers to a metal oxide sol. Metal oxide sols are typically prepared by hydrolysis of a metal alkoxide to form the corresponding metal oxide sol. The term “inorganic non-sol” refers to an inorganic particulate material that is not a metal oxide sol.
In one preferable embodiment of the invention, recyclable material is used as the attrition modifier. This recyclable material may be generated during the manufacture of microspheroidal particles. Microspheroidal particles are conventionally made by spray drying a slurry comprising an inorganic sol and an inorganic non-sol. Typically, a portion of microspheroidal particles made in this manner is not suitable for use as a catalyst support and such a portion is referred to as “recyclable material”. For example, some portion of the microspheroidal particles may be undesirably small, especially in a fluidized bed reactor where they are undesirably carried away from the reactor. It has now been surprisingly discovered that recycled material may be used as an attrition modifier by spray drying a slurry containing this recycled material, an inorganic sol and an inorganic non-sol to form attrition resistant microspheroidal particles.
In another embodiment of the invention, attrition modifier is obtained from recyclable material made during the manufacture of attrition resistant microspheroidal particles. Like other microsphere manufacturing processes, there may be a portion of the attrition resistant microspheroidal particles made by the invention which is not suitable for use as a support catalyst material, i.e. recyclable material. For example, some portions of the microspheroidal particles may be undesirably small, especially in a fluidized bed reactor where unacceptably small particles are undesirably carried away from the reactor. Surprisingly, recycled material from attrition resistant microspheroidal particles may also be used as an attrition modifier by spray drying a slurry comprising this recyclable material, an inorganic sol and an inorganic non-sol.
In accordance with the invention, an attrition resistant catalyst material comprises attrition resistant microspheroidal particles as a catalyst support. Attrition resistant catalyst material may be suitably made by applying a catalytically active component to attrition resistant microspheroidal particles. Typically, the catalytically active component is applied by any method of impregnation known in the art. Suitably, the catalytically active component of the attrition resistant catalyst material may be chosen from metals, metal compounds, organometallic compounds, or mixtures thereof which are known to be catalytically active. Specifically, the catalytically active component is suitably gold, palladium, any other precious metal, or mixtures thereof.
In another embodiment of the invention, attrition resistant catalyst material is used in a fluidized bed reactor. In this embodiment, the catalyst materials are suitably placed in a fluidized state by introduction of a fluidizing medium, usually gas, into the reactor. Reactants, typically in the gas phase, may then be introduced to the reactor where they typically contact the attrition resistant catalyst material in a thorough and uniform manner. Upon contact with the catalyst material, the reactants form a desired product which is carried away from the reactor. During reactor operation, some attrition of catalyst material occurs to create attrited particles, but this amount is advantageously reduced by using attrition resistant catalyst material. Although attritted particles are typically carried away from the reactor along with the product, thereby requiring the separation of these attrited particles from the product by use of a filter, clogging of this filter is advantageously reduced by use of attrition resistant catalyst material.